Exhaust gas purification system of an internal combustion engine and method for purifying the exhaust gases thereof

ABSTRACT

An exhaust gas purification system for purifying the exhaust gas of an internal combustion engine, especially an internal combustion engine featuring auto-ignition and/or direct fuel injection, is provided. The system includes at least one oxidation catalytic converter disposed in an exhaust gas duct of the internal combustion engine, at least one device for selective catalytic reduction of the exhaust gases which is installed downstream of the oxidation catalytic converter, and a feed device for feeding reducing agent into the exhaust gas stream upstream of, or in, the device for selective catalytic reduction. A switch-over device is provided for selectively feeding reducing agent into the exhaust gas stream upstream of, or inside, the at least one oxidation catalytic converter. A corresponding method for purifying the exhaust gas of an internal combustion engine is also provided.

FIELD OF THE INVENTION

The present invention relates to an exhaust gas purification system of an internal combustion engine having a device for selective catalytic reduction, and relates particularly to a method for purifying exhaust gases of an internal combustion engine, in which an exhaust gas stream is passed through a device for selective catalytic reduction.

BACKGROUND INFORMATION

To lower the nitrogen oxide content of oxygen-rich exhaust gas, such as that emitted by diesel internal combustion engines and by internal combustion engines featuring direct gasoline injection, it is known to introduce a reducing agent into an exhaust tract. A suitable reducing agent is, for example, NH₃, which may be introduced as a gas into the exhaust gas stream. In such so-called selective catalytic reduction (SCR), the ammonia is selectively reacted with the nitrogen oxides present in the exhaust gas to form molecular nitrogen and water.

The insufficient activity of the known SCR system at exhaust gas temperatures below approximately 250° C. is a problem. Upstream installation of an oxidation catalytic converter provides, on the one hand, a lowering of the content of deactivating hydrocarbons and, on the other hand, oxidation of NO to NO₂, which leads overall to a marked increase in NO_(x) conversion at exhaust gas temperatures above approximately 200° C. Especially when used in passenger automobiles, however, phases having such low exhaust gas temperatures occur relatively frequently, as is illustrated by a mean catalytic converter temperature of less than 180° C. in the known MVEG test cycle (MVEG: Motor Vehicles Emissions Expert Group; an expert group of the European Commission).

To ensure good distribution of the reducing agent over the SCR catalyst, a mixing section of approximately 40 cm may be provided, with a mixing device, where appropriate. A mixing device for an exhaust gas purification system is described in published German Patent Application 101 31 803.0, which discloses that a mixing body disposed in the exhaust pipe has a gas impingement surface and a jet impingement surface, so that exhaust gas flowing out of the internal combustion engine may impinge upon the gas impingement surface, and reducing agent, which may be fed transversely to the exhaust gas stream, may impinge upon the jet impingement surface.

SUMMARY

An exhaust gas purification system in accordance with the present invention includes at least one oxidation catalytic converter disposed in an exhaust gas duct of an internal combustion engine, and at least one device for selective catalytic reduction of the exhaust gases, which device is installed downstream of the oxidation catalytic converter. The exhaust gas purification system further includes a feed device for feeding reducing agent into the exhaust gas stream and admixing it therewith upstream of the device, or in the device, for selective catalytic reduction (SCR catalytic converter). According to the invention, the exhaust gas purification system has a switch-over device and/or a further feed device for selectively feeding reducing agent into the exhaust gas stream upstream of, or inside, the at least one oxidation catalytic converter. Using such a configuration of the oxidation catalytic converter and the so-called SCR catalytic converter, it is possible to obtain a reduction in the NO_(x) emission to a level below that which ensures compliance with the permissible exhaust gas standards during the MVEG test cycle. Such a reduction in NO_(x) emissions may be achieved by additionally utilizing the temperature-resistant oxidation catalytic converter, which is already present and used for nitrogen oxide oxidation, for the purpose of NO_(x) reduction during a cold start phase. When installed close to the engine, the oxidation catalytic converter will reach a temperature of more than 100° C. after about 50 seconds, which is sufficient for NO_(x) reduction using NH₃ or a reducing agent that splits off NH₃.

Oxidation catalytic converters mainly have noble metals, such as platinum, as the active component. Oxidation reactions of hydrocarbons, carbon monoxide and nitrogen monoxide are thereby promoted even at low temperatures. If NH₃ is injected as the reducing agent, these catalytic converters exhibit a relatively strong NO_(x) reduction activity even at temperatures below 100° C.

If a configuration having a switch-over device instead of a separate feed device for the oxidation catalytic converter is chosen, this may reduce assembly costs. The present invention provides, however, an example embodiment having separate and separately controllable feed devices for reducing agent.

The switch-over device for selectively feeding the reducing agent into the exhaust gas stream upstream of, or in, the oxidation catalytic converter, or into the SCR catalytic converter, may be in the form of a valve, especially a 3/2-way valve. In that manner, the reducing agent may be fed selectively to the oxidation catalytic converter or to the SCR catalytic converter, according to the temperature level that these converters have reached in driving operation.

One embodiment of the invention provides for the switch-over device to be in the form of a mixing valve. In that manner, it is possible for reducing agent to be admitted to the oxidation catalytic converter and the SCR catalytic converter simultaneously during a transition period. Using such a mixing valve, it is possible to avoid an abrupt switch-over, so that, depending upon the operating temperatures reached by the catalytic converters, an optimum purifying effect may be obtained.

The switch-over device may be temperature-controlled, so that, during a cold start phase with exhaust gas temperatures that are still low, reducing agent may be admitted to the oxidation catalytic converter and, after a warm-up phase, to the SCR catalytic converter.

The feed device may include in each case a metering device for quantity metering and nozzles for distributing and atomizing the reducing agent in the exhaust gas stream.

At least one oxidation catalytic converter is disposed in the immediate vicinity of an exhaust gas outlet of the internal combustion engine, with the result that it reaches relatively high temperatures and thus achieves a high purifying effect after only a short time.

The reducing agent may be, for example, an ammonia-containing or ammonia-releasing substance capable of effecting NO_(x) reduction. Examples of such a substance are urea and ammonium carbamate.

In a method according to the present invention for purifying exhaust gases of an internal combustion engine, an exhaust gas stream is passed through at least one oxidation catalytic converter disposed in the exhaust gas duct and through at least one device for selective catalytic reduction (SCR catalytic converter) installed downstream of the oxidation catalytic converter, and a reducing agent is selectively fed to the exhaust gas stream upstream of, or inside, the at least one oxidation catalytic converter. The reducing agent may be selectively fed to both catalytic converters simultaneously or to only one of the catalytic converters. The reducing agent may be distributed and atomized by a nozzle.

One embodiment of the method according to the present invention provides for temperature-controlled feeding of the reducing agent into the oxidation catalytic converter and/or into the device for selective catalytic reduction.

If NH₃ is admitted to the oxidation catalytic converter, the latter exhibits a relatively pronounced NO_(x) reduction activity at temperatures below 100° C. The useful temperature window for NO_(x) reduction is relatively narrow, however, since above approximately from 250° C. to 300° C. nitrogen reduction no longer takes place but, rather, additional nitrogen oxide production takes place as a result of oxidation of NH₃. In addition, relatively high N₂O selectivities may possibly be observed. It must, therefore, be ensured that reducing agent is admitted to the oxidation catalytic converter only in a starting phase (in the MVEG test, only up to about 350 s). For example, the reducing agent is fed into the oxidation catalytic converter at exhaust gas temperatures of less than approximately from 150° C. to 200° C. in the oxidation catalytic converter.

After such a period of time, the SCR catalytic converter will normally also have reached its operating temperature and injection of reducing agent is switched to the SCR catalytic converter. That may be done at temperatures of approximately from 150° C. to 200° C. in the SCR catalytic converter. Metering of reducing agent onto the oxidation catalyst is possible in principle at operating points with a low exhaust gas temperature—that is to say, not only in the case of cold starting—and provides a very effective NO_(x)-lowering potential in cases where only insufficient activity is achieved with the SCR catalytic converter. With injection upstream of the oxidation catalytic converter up to a time of about 600 s, therefore, a marked increase in conversion by the exhaust gas purification system may be achieved. A suitable, sensible switch-over point of the temperature-controlled switch-over valve may lie at 100 to 200° C., e.g., at 130 to 180° C.

An example embodiment of the system according to the present invention may, for example, provide a 3/2-way switch-over valve which is operated in dependence upon the catalytic converter temperatures and the operating point of the engine. Equipping an existing system in that manner is relatively simple and may be done with only little expenditure. The catalyst system, the temperature sensors and the metering system are already present, and only the switch-over valve and the reducing agent feed line upstream of the oxidation catalytic converter need to be implemented. By a suitable metering strategy, it is possible for effective reduction of the nitrogen oxides to be achieved over an entire test cycle (MVEG cycle). In the MVEG test, it is possible for an increase in NO_(x) conversion of approximately 40% to be achieved, with the result that in the case of lowered untreated emissions it is even possible to meet the relatively strict U.S. standards.

The oxidation catalytic converter may, in an example embodiment, be in the form of a catalytically coated particle filter. The catalytic coating of the particle filter acts in this case similarly to the coating of an oxidation catalytic converter. It is furthermore possible for a separate particle filter, which effects filtering of the soot particles, to be provided between the oxidation catalytic converter and the SCR catalytic converter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an internal combustion engine having an exhaust gas after-treatment unit in an exhaust gas duct.

FIG. 2 is a schematic illustration of the internal combustion engine according to FIG. 1 in a first operating position.

FIG. 3 shows the internal combustion engine according to FIG. 1 in a second operating position.

FIG. 4 is a chart showing typical temperature variations of the components of the exhaust gas purification system during a test cycle.

FIG. 5 is a chart showing typical NO_(x) emission values during a test cycle.

FIG. 6 is a chart illustrating the SCR activity of an oxidation catalytic converter.

DETAILED DESCRIPTION

FIG. 1 shows an exhaust gas purification system according to the present invention. In FIG. 1, an oxidation catalytic converter 4 and a device for selective catalytic reduction, referred to as SCR catalytic converter 6, are disposed in an exhaust gas duct 28 of an internal combustion engine 2. Internal combustion engine 2 has an intake duct 21 for supplying fresh mixture 22, and outlet ducts 26 which are combined in a manifold 27 to form exhaust gas duct 28. Disposed in the exhaust gas duct is an exhaust gas turbine 24 of an exhaust gas turbocharger 23, which turbine 24 is coupled via a shaft 25 to a compressor, not shown here. Exhaust gas turbocharger 23 is optional and serves to improve the performance and exhaust gas emission characteristics of internal combustion engine 2.

Internal combustion engine 2 may be a diesel internal combustion engine featuring auto-ignition or a gasoline engine featuring direct fuel injection. Both types of engine emit a relatively oxygen-rich exhaust gas. Exhaust gas stream 29 passes successively through oxidation catalytic converter 4 and SCR catalytic converter 6 and leaves the exhaust gas purification system as purified exhaust gas 14 which is passed into the open air via a muffler (not shown). The exhaust gas purification system further includes a feed device 8 for feeding a reducing agent 81 into exhaust gas stream 29. Feed device 8 includes a switch-over device 83 and also a first connection line 84, which is connected to a first nozzle 85, and a second connection line 86, which is connected to a second nozzle 87. First nozzle 85 is disposed upstream of oxidation catalytic converter 4 in exhaust gas duct 28 and serves to finely distribute and atomize reducing agent 81 upstream of oxidation catalytic converter 4. Second nozzle 87 is disposed upstream of SCR catalytic converter 6 and downstream of oxidation catalytic converter 4 and serves to feed reducing agent 81 into exhaust gas stream 29 upstream of SCR catalytic converter 6.

First and second connection lines 84, 86 open into switch-over device 83 which is able to provide for selective distribution of the reducing agent to first and/or second connection line 84, 86. Switch-over device 83 may be controlled temperature-dependently, so that, in a cold running phase, reducing agent 81 may be admitted to oxidation catalytic converter 4 and, after a certain temperature has been reached, to SCR catalytic converter 6.

FIG. 2 illustrates the cold running phase of the exhaust gas purification system, in which reducing agent is admitted only to first nozzle 85. This is illustrated by arrow 81 along first connection line 84.

FIG. 3 illustrates the subsequent phase, in which the catalytic converters have already reached a predetermined operating temperature. In this case, reducing agent 81 is admitted to second connection line 86 and second nozzle 87. This is illustrated by arrow 81 along second connection line 86.

A typical transition temperature may lie at approximately from 100° C. to 200° C., e.g., at about from 130 to 180° C., above which a switch-over to admission of reducing agent 81 to SCR catalytic converter 6 may take place. A switch-over may also be made in an advantageous manner by a mixing valve, which is able to provide for simultaneous admission to oxidation catalytic converter 4 and SCR catalytic converter 6 in the transition temperature range.

An alternative configuration according to the present invention provides two separate feed devices for the oxidation catalytic converter and the SCR catalytic converter.

In an alternative embodiment, oxidation catalytic converter 4 may be a catalytically coated particle filter which, by virtue of its catalytic coating, has the same effect as an oxidation catalytic converter. In addition to the configuration shown, a separate particle filter may be disposed between oxidation catalytic converter 4 and SCR catalytic converter 6. That particle filter produces a further improvement in the purifying effect on the exhaust gases.

FIG. 4 illustrates typical temperature variations of the oxidation catalytic converter and the SCR catalytic converter during a standardized test cycle. The so-called MVEG test will be referred to hereinafter as an example of a test cycle. The time in seconds is plotted on the horizontal axis and the temperature in ° C. on the vertical axis. It will be apparent that the oxidation catalytic converter (upper, jagged curve) is capable of reaching temperatures of up to 200° C. after a period of as little as approximately 150 seconds. The temperature of the SCR catalytic converter (lower, undulating curve) is still distinctly below 150° C. after 300 seconds. At those temperature ranges in the SCR catalytic converter, feeding of reducing agent will not yet produce satisfactory reduction results for NO_(x). Since the oxidation catalytic converter reaches temperatures of more than 100° C. after only a few seconds, it is possible for good NO_(x) reduction to be already achieved by feeding reducing agent upstream of, or into, the oxidation catalytic converter. The dashed vertical line at approximately 300 seconds represents the earliest sensible time to commence NH₃ injection upstream of the SCR catalytic converter. The continuous vertical line at approximately 350 seconds represents the start of effective NO_(x) reduction by the SCR catalytic converter in the MVEG test cycle.

FIG. 5 illustrates the cumulative emission of NO_(x) over time in various systems for exhaust gas purification. The time in seconds is shown on the horizontal axis and the cumulative amount of emitted NO_(x) is shown on the vertical axis. It will be apparent that, by injecting reducing agent upstream of the oxidation catalytic converter and in the SCR catalytic converter in accordance with the present invention, it is possible for emissions of NO_(x) to be markedly reduced.

Lowermost curve 20 illustrates that only with the system according to the present invention is it possible to comply with the MVEG limit value of 0.9 g of NO_(x). Discontinuous curve 22 extending above the latter characterizes the curve for NO_(x) emissions in a conventional system composed of oxidation catalytic converter and SCR catalytic converter arranged in series (so-called conventional VR system without switch-over). Curve 24 illustrates the emissions of a system that provides feeding of reducing agent merely upstream of the oxidation catalytic converter. At first, good reduction takes place, but the elevated temperatures at and beyond approximately 800 seconds prevent effective NO_(x) reduction. From that point in time, the NO_(x) emissions rise steeply and even approach the values of untreated emissions (curve 26), since, at and above approximately from 300 to 350° C., an additional quantity of NO_(x) is produced.

FIG. 6 illustrates the NO_(x)-reducing effect of the oxidation catalytic converter as a function of temperature. It will be apparent that, at and above a temperature of approximately 200° C., NO_(x) reduction falls markedly and that, at and above temperatures of approximately 350° C., NO_(x) is even additionally produced. The temperature is shown on the horizontal axis and the conversion is shown on the vertical axis. It will be apparent that the conversion of NO_(x) declines markedly at and above a certain temperature (approximately 200° C.). That is the reason why, after the cold running phase, the feeding of reducing agent upstream of the oxidation catalytic converter should be discontinued and reducing agent may continue to be fed only upstream of the SCR catalytic converter. 

1-16. (canceled)
 17. An exhaust gas purification system for purifying an exhaust gas stream of an internal combustion engine, comprising: at least one oxidation catalytic converter provided in an exhaust gas duct of the internal combustion engine; at least one device for selective catalytic reduction of the exhaust gas stream, the at least one device being provided downstream of the at least one oxidation catalytic converter; and a feed device for feeding a reducing agent into the exhaust gas stream one of upstream of the at least one device for selective catalytic reduction and in the at least one device for selective catalytic reduction; and a switch-over device for selectively feeding the reducing agent into the exhaust gas stream one of upstream of the at least one oxidation catalytic converter and in the at least one oxidation catalytic converter.
 18. The exhaust gas purification system as recited in claim 17, wherein the switch-over device is a valve.
 19. The exhaust gas purification system as recited in claim 18, wherein the switch-over device is a directional control valve.
 20. The exhaust gas purification system as recited in claim 18, wherein the switch-over device is a mixing valve.
 21. The exhaust gas purification system as recited in claim 19, wherein the switch-over device is temperature-controlled.
 22. The exhaust gas purification system as recited in claim 20, wherein the switch-over device is temperature-controlled.
 23. The exhaust gas purification system as recited in claim 21, wherein the feed device has a metering device and a nozzle for distributing and atomizing the reducing agent in the exhaust gas stream.
 24. The exhaust gas purification system as recited in claim 22, wherein the feed device has a metering device and a nozzle for distributing and atomizing the reducing agent in the exhaust gas stream.
 25. The exhaust gas purification system as recited in claim 23, wherein the at least one oxidation catalytic converter is disposed in the immediate vicinity of an exhaust gas outlet of the internal combustion engine.
 26. The exhaust gas purification system as recited in claim 25, wherein the at least one oxidation catalytic converter is a catalytically coated particle filter.
 27. The exhaust gas purification system as recited in claim 25, further comprising: at least one particle filter provided between the at least one oxidation catalytic converter and the at least one device for selective catalytic reduction.
 28. A method for purifying an exhaust gas stream of an internal combustion engine, comprising: passing the exhaust gas stream through at least one oxidation catalytic converter located in an exhaust gas duct of the internal combustion engine, and through at least one device for selective catalytic reduction located downstream of the at least one oxidation catalytic converter; and selectively performing at least one of: a) feeding a reducing agent into the exhaust gas stream one of upstream of the at least one device for selective catalytic reduction and in the at least one device for selective catalytic reduction; and b) feeding the reducing agent to the exhaust gas stream one of upstream of the at least one oxidation catalytic converter and in the at least one oxidation catalytic converter.
 29. The method as recited in claim 28, wherein the reducing agent is fed into one of the at least one oxidation catalytic converter and the at least one device for selective catalytic reduction.
 30. The method as recited in claim 28, wherein the reducing agent is fed into the at least one oxidation catalytic converter and the at least one device for selective catalytic reduction simultaneously during a transition period.
 31. The method as recited in claim 28, wherein the reducing agent is fed by a nozzle.
 32. The method as recited in claim 28, wherein the selective feeding of the reducing agent is determined as a function of temperature.
 33. The method as recited in claim 30, wherein the reducing agent is fed into the at least one oxidation catalytic converter at an exhaust gas temperature of less than approximately 180° C. in the at least one oxidation catalytic converter.
 34. The method as recited in claim 32, wherein the reducing agent is fed into the at least one oxidation catalytic converter at an exhaust gas temperature of less than approximately 180° C. in the at least one oxidation catalytic converter.
 35. The method as recited in claim 33, wherein the reducing agent is fed into the at least one device for selective catalytic reduction at an exhaust gas temperature of more than approximately 180° C. in the at least one device for selective catalytic reduction.
 36. The method as recited in claim 34, wherein the reducing agent is fed into the at least one device for selective catalytic reduction at an exhaust gas temperature of more than approximately 180° C. in the at least one device for selective catalytic reduction. 